1. Field of the Invention
Embodiments of the present invention relate generally to memory devices, and, more specifically, to digital filters with memory for reading from, and/or writing to, memory elements in memory devices.
2. Description of the Related Art
Generally, memory devices include an array of memory elements and associated sensing circuits. The memory elements store data, and the sensing circuits read the data from the memory elements. To read data, for example, a current is passed through the memory element, and the current or a resulting voltage is measured by the sensing circuit. Conventionally, the sensing circuit measures the current or voltage by comparing it to a reference current or voltage. Depending on whether the current or voltage is greater than the reference, the sensing circuit outputs a value of one or zero. That is, the sensing circuit quantizes or digitizes the analog signal from the memory element into one of two logic states.
The sensing circuit also provides feedback when writing to the memory element. In some memory devices, writing is an iterative process in which a value is written to the memory element by incrementally changing some property of the memory element, such as charge stored on a floating gate. After each iteration, the sensing circuit reads from the memory element to determine whether the changed property reflects the target value to be written to the memory element. If the property indicates the proper value, then the process of incrementally changing the property stops. Otherwise, the property is changed by another increment, and the sensing circuit reads from the memory element, repeating the process until the memory element stores the target value. Thus, each time data is written to the memory element, the sensing circuit may both read from the memory element and compare the resulting value to a target value several times.
Certain conventional sensing circuits can slow the writing process. These sensing circuits request and receive the target value over an input/output bus each time that they compare the target value to the value stored by the memory element. Acquiring the target value over the input/output bus can take several clock cycles. As a result, these sensing circuits may increase the time between each iteration of the writing process and, as a result, slow the operation of the memory device.
Additionally, some conventional sensing circuits include comparison circuitry that increases the size of memory devices, which tends to increase their cost. Certain conventional sensing circuits include comparison circuitry that, during a write operation, compares the target value to the value stored by the memory element. For multi-bit memory elements, the comparison circuitry may compare each digit of a multi-bit target value to each digit of a multi-bit value stored by the memory element. Circuitry configured to compare each digit may consume valuable chip surface area, especially in sensing circuits designed to sense multi-bit memory elements.